Data

Data | How lesser rainfall, reduced inflows & measured water release from Chembarambakkam blunted Cyclone Nivar's impact

Good flow: Water released from the Chembarambakkam reservoir seen flowing in Adyar river near Saidapet. K.V. Srinivasan  

Cyclone Nivar, which made landfall in the early hours of November 26, seemed far less destructive compared to earlier cyclones that swept the coast of Tamil Nadu.

Some reasons why the impact was minimised could be a distributed rainfall pattern, reduced inflow of water into the Chembarambakkam reservoir and better management of the outflow compared to the 2015 floods.

Battered coast

The coloured dots in the map correspond to the landing points of recent cyclones that hit Tamil Nadu’s coastline. The bigger the dots and the deeper the red, the higher the cyclone's speed at the time of landfall.

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1. Cyclone name: Thane | Date of landfall: Dec. 30, 2011 | Approximate speed during/just before landfall (knots): 75 | Location: Close to north of Cuddalore.

2. Gaja | Nov. 15, 2018 | 70 knots | Between Nagapattinam and Vedaranyam.

3. Nivar | Nov. 25, 2020 | 70 knots | Between Mamallapuram and Karaikal.

4. Unnamed | Nov. 29, 2000 | 60 knots | Near Cuddalore.

5. Vardah | Dec. 12, 2016 | 60 knots | Near Chennai port.

6. Nisha | Nov. 27, 2008 | 45 knots | North of Karaikal.

7. Nilam | Oct. 31, 2012 | 45 knots | Near Mamallapuram, south of Chennai.

8. Fanoos | Dec. 10 2005 | 35 knots | Close to Vedaranyam.

9. Jal | Nov. 7, 2010 | 30 knots | Close to North Chennai.

10. Madi | Dec. 12, 2013 | 25 knots | Near Vedaranyam.

11. Nada | Dec. 2, 2016 | 25 knots | Near Nagapattinam.

Rainfall pattern: Less intense and more spread out

One possible reason why Nivar seemed far less destructive than many previous cyclones was the fact that the rainfall was distributed over three days and did not pour down in short and intense spells.

A comparison with the rainfall over Mumbai during the September 2019 floods helps understand the difference. The second graph shows rainfall recorded in Nungambakkam station, every 15 minutes, starting from 9.15 p.m. on Nov. 23, 2020 to 1.30 p.m. on Nov. 26, 2020. The first graph shows rainfall recorded in Mumbai’s Santa Cruz station, every 15 minutes, starting from 9.30 a.m. on Sep. 2, 2019 to 1.45 a.m. on Sep. 5, 2019.

Both periods amount to 64 hours and 15 minutes in total. In the considered period, Nungambakkam recorded 320.5 mm of rainfall, lower than that of Santa Cruz’s 476 mm. Thus, the overall intensity was lower.

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